It has been known for some time that phosphate ores with siliceous gangues can be enriched by flotation. In accordance with the prior art, the usual processes for enrichment by flotation consist of subjecting the phosphate ores with siliceous gangues to a first flotation utilizing fatty acids; in the froth there is obtained the phosphate and part of the silica of the gangue. This froth is washed with sulfuric acid to eliminate the traces of fatty acids remaining on the phosphate particles and on the silica and silicate grains. The washed product is subjected to a second flotation performed utilizing amines: the silica and silicates are recuperated in the froth while a phosphate concentrate is obtained in the tailings.
However, these known processes are not applicable to separation of the phosphates and carbonates present in phosphate ores with carbonate gangues, firstly because of the similar electrochemical behavior of phosphate and carbonate particles and secondly because of the similar reaction of carbonates and phosphates to the presently-known collectors. In other words, if known processes of enrichment by flotation of phosphate ores with siliceous gangues are applied to flotation of phosphate ores with carbonate gangues, the phosphates are not separated from the carbonates since they float together.
It emerges from the above that, thus far, no success has been had in satisfactorily separating phosphate ores from the carbonate gangue by flotation; research performed thus far has not enabled a flotation enrichment process to be developed for these types of ores. Separation processes for phosphate ores with carbonate gangues used today consist of calcination, generally followed by elimination of the lime produced and sometimes by manufacture of elementary phosphorus. These techniques however require complex and costly installations and therefore have a high cost price.
Furthermore, enrichment processes for phosphate ores with carbonate gangues using calcination apply only to certain types of phosphate ores, namely phosphate ores whose gangues contain predominantly carbonates of alkaline earth metals, excluding ores containing, together with such carbonates, relatively substantial proportions of silica and silico-aluminates which, during calcination, form combinations with the carbonate which cannot be eliminated from the phosphate and which make the product obtained difficult to market due to the impurities it contains.
Flotation processes have a number of advantages, in particular their simplicity of operation, the opportunity they provide of processing considerable tonnages, and, above all, their very low cost price; this is the reason for which it proved desirable to develop a technique able to permit application of flotation processes to enrichment of phosphate ores with carbonate gangues.